WO-A-01/98393 describes 2K (2-component) coating compositions comprising a polyol binder component and a polyisocyanate crosslinker component partly functionalized with alkoxysilyl groups. These coating compositions are used as primers and are optimized for adhesion to metallic substrates, especially aluminum substrates. Over these coating compositions, as part of an OEM finish or a refinish, it is possible to apply basecoat/clearcoat systems. In terms of scratch resistance and weathering stability, the coating compositions of WO 01/98393 are not optimized.
EP-A-0 994 117 describes moisture-curable mixtures comprising a polyol component and a polyisocyanate component which may partly have been reacted with a monoalkoxysilylalkylamine that has undergone reaction preferably to an aspartate. Although coatings formed from such mixtures do have a certain hardness, they are nevertheless of only limited suitability for OEM applications in terms of their weathering stability and, in particular, their scratch resistance.
US-A-2006/0217472 describes coating compositions which can comprise a hydroxy-functional acrylate, a low molecular mass polyol component, a polyisocyanate, and an amino-functional alkoxysilyl component, preferably bisalkoxysilylamine. Such coating compositions are used as clearcoat material in basecoat/clearcoat systems and lead to scratchproof coatings. Coating compositions of this kind, however, have only very limited storage qualities, and the resulting coatings have low weathering stability, particularly with respect to UV radiation in a wet/dry cycle.
WO 2006/042585 describes clearcoat materials which are suitable for OEM finishing and which as their main binder component comprise polyisocyanates whose isocyanate groups, preferably to an extent of more than 90 mol %, have undergone reaction with bisalkoxysilylamines. Clearcoat materials of this kind combine excellent scratch resistance with high chemical and weathering resistance. But there is still a need for a further improvement in the weathering stability, particularly with respect to cracking under UV irradiation in a wet/dry cycle, with retention of the high level of scratchproofing.
EP-A-1 273 640 describes 2K coating compositions composed of a polyol component and of a crosslinker component consisting of aliphatic and/or cycloaliphatic polyisocyanates, 0.1 to 95 mol % of the free isocyanate groups originally present having undergone reaction with bisalkoxysilylamine. These coating compositions can be used for OEM finishing and, after their curing is complete, combine good scratch resistance with effective resistance to environmental influences. Nevertheless, these coating compositions have a particularly strong propensity toward aftercrosslinking, which—straight after the final thermal curing only results in inadequate scratch resistance of the coatings. The strong post-crosslinking likewise has a negative impact on the weathering stability since it leads to an increased risk of cracking under pressure.
The as yet unpublished German patent application P102007013242 describes coating compositions which comprise surface-actively modified, silane-containing compounds. These coating compositions lead to finally cured coatings which have a higher density of Si atoms of the Si—O—Si network in the near-surface coating zone—owing to the accumulation of the surface-actively modified, silane-containing compounds prior to thermal curing—than in the bulk. This accumulation of the Si—O—Si network at the surface is specifically not exhibited by the coatings of the invention; instead, the regions of the Si—O—Si network of the finally cured coating of the invention are distributed statistically.
It was an object of the present invention to provide coating compositions, particularly for the clearcoat film in OEM finishes and automotive refinishes, that lead to a network with a high degree of weathering stability, the unwanted formation of moieties unstable to hydrolysis and weathering being very largely suppressed, in order to ensure high acid resistance. In addition, the coating compositions ought to lead to coatings which already have a high degree of scratchproofing straight after thermal curing and in particular a high retention of gloss after scratch exposure. Moreover, the coatings and coating systems, especially the clearcoat systems, ought to be able to be produced even in film thicknesses >40 μm without stress cracks occurring. This is a key requirement for the use of the coatings and coating systems, particularly of the clearcoat systems, in the technologically and esthetically particularly demanding field of automotive OEM finishing.
The intention in particular was to provide clearcoat systems featuring high resistance, particularly to cracking, under weathering with UV radiation in a wet/dry cycle, in combination with outstanding scratchproofing.
Furthermore, the new coating compositions ought to be preparable easily and with very good reproducibility, and ought not to present any environmental problems during application of the coating material.